This invention relates to photographic objectives of the standard or quasi-standard angular field, and more particularly to objectives of a large aperture ratio with a diaphragm in the image space.
Lens shutter type cameras require standard objectives to be of small bulk and size with an angle of view exceeding 54.degree.. A type of lens which has gained general acceptance for an objective having a relative aperture as large as F/2, is the diaphragm interspaced Gauss type. An example of a modified Gauss type objective is disclosed in British Pat. No. 865,530.
These diaphragm-interspaced Gauss type objectives through producing more or less flare in the intermediate to the marginal region of the image field have been proven to give good correction of field curvature for high grade imaging performance throughout the entire area of the image field.
At the present time, on the other hand, photo-electric sensing of an object distance for automatic adjustment of the objective, or so-called auto-focus mechanisms has become practical, thereby freeing many photographers from troublesome focusing operation.
Since this mechanism is powered by a tensioned spring or an electric motor as the drive source to effect focusing, the employment of the diaphragm interspaced Gauss type in the lens design leads to the necessity of moving along with the lens system, the large mass of the diaphragm device associated with a shutter and other mechanical parts. This gives rise to disadvantages, the first of which is that a strong torque is required to assure the satisfactory operation, and second is that the diaphragm, shutter and auto-focus mechanism are also made complicated. These problems have contributed to the difficulty of minimizing the bulk and size of the camera.
Arrangement of the shutter diaphragm behind the objective lens system is an effective way to solve these problems. But, as the objective becomes faster with the F-number down to as low as 2, use of the rear diaphragm arrangement creates various aberrational problems. To avoid the reduction of the image quality, therefore, a sacrifice must be made, for example, by narrowing the angular field and elongating the physical length. Even in the improved conventional objectives such as that shown in Japanese Patent Sho 41-15463 (1966), the total length from the front vertex to the image plane is limited to 1.23 times the focal length or longer, and the angle of view to 52.degree. or less.